In this project we propose to study the structure of human tyrosine accepting transfer RNAs and the biosynthesis of pseudouridine in mammalian cells. Tyrosine-accepting transfer RNAs will be isolated and purified from human placental tissues and from cultures of HeLa cells. The structure of these RNAs will be analyzed systematically and compared with each other. The analysis will include determinations of nucleotide compositions for each species of tRNA preparation and comparison of oligonucleotide "fingerprints", and ultimately, comparisons of nucelotide sequences of these structures. Analysis of pseudouridine biosynthesis will include: 1) a survey of tRNA pseudouridylation enzymes in normal, differentiating, and neoplastic cells: 2) purification of these enzymes from some representative sources: 3) examination of the site specificities and other kinetic and structural properties of the enzymes: and 4) screening of various tissue extracts for modulators of pseudouridine synthetase activity.